The present invention is directed to a process and device for detecting print registration marks on a multi-color printing press processing sheet or web-shaped matter having an area for printing an image and an area for the printing of accuracy control signs or marks.
The problems, which are involved with the registration color prints of various units of a multi-color print press, especially for varying printing units of an offset press, are well known to the designer of such machines. These designers have already investigated into ways of detecting and correcting misregistration in order to avoid, as reliably as possible, the print deficiency caused by misregistration of the various color prints.
One of the solutions adopted consists in printing one or several registration marks as a reference for the image printed and these marks are situated within a printless area of the sheet or web-like matter to be processed by the printing machine. For instance, these marks are placed on the margin of the sheet or web or else, in certain cases, within a space situated between two successive prints. These registration marks, which are used for checking both lengthwise and crosswise misregistration, usually have a trapezoidal shape and their detection is achieved by a scanning device. Checking the position of a registration mark printed by a printing unit with reference to the position of a registration mark printed by another printing unit, therefore, requires, in the first place, the opening of a scanning window in the area foreseen for the arrangement of the registration mark. This requires a determination of the position of such a scanning window and the recognition therein of an image which will actually be the registration mark. The subsequent operation consists in measuring the mark dimensions within an area identical to all subsequent marks appearing under the scanning device so that the existing misregistration can be obtained. At the next stage, the measuring data is converted into corrective signals, which are used by a registration correction appliance. With this solution, the registration marks have a relatively large size enabling proper scanning. A device which allows the obtaining of a scanning window is described in detail by Swiss Patent No. 548,933. A device for scanning and recognition of register marks is covered and described in Swiss Patent No. 556,738.
As already mentioned, this way requires large size marks to insure acceptable detection of crosswise and lengthwise print misregistration. However, the acceptance of large size marks will inevitably lead to a heavy loss of material. In fact, on account of the present printing technology, registration marks are required to be situated in a printless area which is rather large to enable reliable distinction from the printed motif. Furthermore, the area for printing registration marks is not to subsist on the final product, for which reason it is to be eliminated as a mere piece of waste.